Publication costs assisted try Colorado State UniversityPhotosynthetic production of H2 and H202 is demonstrated on UV-irradiated ZnO/water and Ti02/water suspensions. The rates of H2 and H202 production were monitored by a combined analytical-gas chromatographic procedure as a function of irradiation time and solid content. In the case of ZnO, evidence is presented which indicates that photocorrosion of ZnO proceeds simultaneously with photosynthesis of H2 and HzOD The present data are rationalized with the aid of energy band diagrams showing the relative positions of the semiconductor band edges and the solution redox levels. IntroductioinThere has lbeen much recent interest in photocatalytic and photosynthetic processes at the a,emiconductor/liquid interface-l Interest in photosynthetic processes stems primarily from the possibility of utillizing these reactions to convert sunlight to storable forms of energy. Two reactions which have received particular attention in this regard are Both of these reactions are thermodynamically "uphill" and involve respectively free-energy changes of 2.43 and 1.26 eV per molecule. Photoelectrochemical (PEC) methods h a w offered a ready means of overcoming the thermodynamic barriers and have einabled production of fuels by reactions 1 and 2. Several rleviews of this field of research have appeared in the recent literaturea2 An attractive alternative to the PEC approach involves the socalled photoriynthetic m0de.l' This approach utilizes semiconducting grains which are suspended in the appropriate reaction media and thereby affords a more practical means of carrying out the energy conversion process. Each illuminated grain may be regarded as a short-circuited PEC cell, and the reactions at the interface of each grain and the liquid medium constitute the "local cell"ld,fJ processes.Photosynthetic production of Hz02 according to reaction 2 has been investigated on various semiconducting materials including ZnO?s5 CdS,4 HgS: ZnS,4 CdTe,6 CdSe,4 and Ga2S3.4 ZnO, in particular, has been the focus of considerable a t t e n t i~n .~,~ Notwithstanding the controversies that remain on the mechanism of photosynthesis of Hz02 at the ZnO/liquid i~i t e r f a e e ,~~ it was particularly of interest to note that previous authors had reported no traces of hydrogen in the reaction products.3c In view of the importance of reaction 1 to energy conversion and storage and the preponderance of this reaction at oxide semiconductor/electrolyte interfaces in general (vide infra), we decided to reexamine the products of UV-generated processes at the powdered ZnO/liquid interface.A second aspect of relevance to this paper concerns reaction 2. Pirevious authors had observed H202 production at the Ti02/electrolyte interface subjected to UV and visible irradiation.6 These experiments were, however, conducted in the photoelectrochemical mode. With powdered Ti02 grains suspended in aqueous solution, no evidence for reaction 2 was obtained in previous studies.3a This finding was surprising particularly in vie...

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State-of-the-art SOECs consisting of a nickel-yttria stabilized zirconia (Ni-YSZ) fuel electrode, YSZ electrolyte and lanthanum strontium cobaltite ferrite-gadolinium doped ceria (LSCF-GDC) composite oxygen electrode were tested under co-electrolysis (H 2 O+CO 2 ) conditions. The aim in this study was to compare the SOEC durability under co-electrolysis conditions between galvanostatic and potentiostatic modes. Specifically, the cells were operated at 0.75 A/cm 2 (galvanostatic) and at 1.2 V (potentiostatic) at 750 • C for over 1000 hours. In both modes, a larger degradation was observed initially for the first 200 hours of testing, followed by a more stable performance over longer operating times. Interestingly, there was a difference in trends of serial and polarization resistances' evolution. In galvanostatic mode of operation, both increased while for potentiostatic mode only the polarization resistance increased over time. The difference of the degradation was attributed to the overpotentials being experienced by the cells in the respective modes. Trends of the area specific resistance (ASR) and detailed electrochemical analysis of the performance of the cell under durability conditions for both modes indicated that the degradation was due to both the fuel electrode and the oxygen electrode, with an additional contribution from fuel electrode in galvanostatic testing. Microstructural analysis also confirmed the degradation of the active fuel electrode.

An electrolytic technique utilizing the cell, Pt]WlGa, fl-Ga20sI calcia stabilized zirconiaIGa-In, /~-Ga2031WIPt, has been used to measure activities of Ga in liquid In-Ga alloys in the temperature range 800~176The Ga activities and the In activities, calculated from the Gibbs-Duhem equation, show modest positive deviation from Raoult's law. The activities are compared with previously measured and computed values. The parameter acaIn (defined by In 7ca/[1 --Naa] 2) is seen to have a very small composition dependence in the temperature range studied. The partial molar excess entropies of solution of Ga, calculated using the measured free energies and previously measured values for the heats of mixing, are demonstrated to be negative and small.

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